There’s a dedicated page in the wiki that you can edit to report spelling mistakes in the game’s original English strings. The main reason to have this list is to be able to keep track of mistakes that need to be fixed after a string freeze is over. Naturally, grammar mistakes also belong to this page.

Due to the prose style favored in mainline Wesnoth, non-native speakers may occasionally come across words and constructions that may sound or look like invalid English, but are actually intended and correct forms not from Modern English. You should check the following page before reporting mistakes in the wiki above.